JP2005063509A - Magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a magnetic recording medium which has excellent recording current characteristics and frequency characteristics and high output and whose power consumption is reduced. <P>SOLUTION: The magnetic recording medium has: a non-magnetic layer 3 containing inorganic powder and a binder; and a magnetic layer 2 containing ferromagnetic powder and a binder, both layers being formed on one principal surface of a non-magnetic substrate 1; and a back layer 4 formed on the other principal surface. In the magnetic recording medium, coercive force Hc and SFD (switching field distribution) have the relation of 220≤Hc×(1+0.5×SFD), and the coercive force Hc and a squareness ratio Rs have the relation of 2.2≤Hc/Rs≤2.6. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a high-density magnetic recording medium particularly suitable for recording large-capacity data.

  Magnetic recording technology has advantages such as the ability to repeatedly use recording media and lower running costs compared to other recording media, so it can be used in video, audio, computer applications, etc. It is widely used in various fields.

  In recent years, various studies have been made to reduce the thickness of magnetic recording media and improve the recording density in order to meet the demands for reducing the size of equipment, improving the quality of recording / reproducing signals, and increasing the recording capacity. ing.

  With regard to the improvement of recording density, for example, the compression of recorded data is generally performed by the advancement of video signal compression technology. At the same time, the data compression rate is relatively low and high signal quality is achieved. There is an increasing demand for so-called high-quality video signals that are maintained.

Methods for improving the recording density include shortening the wavelength to shorten the length of the recording data signal per unit bit and narrowing the track to narrow the writing width.
For example, in the HDCAM format of 1/2 inch digital video manufactured by Sony, the track width is about 20 μm, and the length of the recording data signal per unit 2 bits is about 0.5 μm. The data transfer rate is 140 Mbps on the video network.

  In view of the above, for example, in order to realize a transfer rate of 300 Mbps or higher in a video net, it can be said that it is an essential problem to greatly increase the density of a magnetic recording medium. For that purpose, it is necessary to greatly shorten the wavelength and narrow the track of the recording data signal. On the other hand, shortening the wavelength and narrowing the track reduce the reproduction output of the recording data signal and the S / N. Has the problem of being invited.

Conventionally, as a high-density type magnetic recording medium that improves the transfer rate and at the same time realizes good electromagnetic conversion characteristics, a medium that defines the thickness of the magnetic layer and the magnetic flux of the magnetic layer has been proposed (for example, , See Patent Document 1).
However, it cannot be said that this conventionally proposed technique has high adaptability in a practical magnetic recording / reproducing system. That is, in terms of a practical aspect regarding the magnetic recording medium, the magnetic recording medium while taking into consideration the balance between the recording current and the reproduction output, that is, the margin of the recording current value and the optimum recording current for obtaining sufficient reproduction output. However, depending on the technology disclosed in Patent Document 1 below, the above-described practical characteristics have not yet been studied.

JP 11-238225 A

  Therefore, in the present invention, in view of the problems of the prior art as described above, the magnetic recording medium is studied from a practical aspect, the electromagnetic conversion characteristic is good, and a large capacity capable of realizing a stable error rate characteristic. Type magnetic recording medium.

In the present invention, on one main surface of the nonmagnetic support, a lower nonmagnetic layer containing inorganic powder and a binder, and a magnetic layer containing ferromagnetic powder and a binder are formed, A magnetic recording medium having a total thickness of 12 μm or less having a configuration in which a back layer is formed on another main surface, and the coercive force Hc and SFD (switching field distribution) are represented by the following formula (1): There is provided a magnetic recording medium in which the coercive force Hc of the magnetic layer and the squareness ratio Rs have the relationship of the following formula (2).
220 ≦ Hc × (1 + 0.5 × SFD) (1)
2.2 ≦ Hc / Rs ≦ 2.6 (2)

  According to the present invention, the recording current characteristic for obtaining a practically sufficient reproduction output can be improved, and a sufficient reproduction output can be obtained without increasing the recording current value.

  In a magnetic recording medium having a total thickness of 12 μm or less, in which a nonmagnetic layer and a magnetic layer are laminated on a nonmagnetic support, the coercive force Hc and SFD of the magnetic layer satisfy the relationship of the above formula (1). As a result, good overcurrent characteristics were obtained in the recording current vs. reproduction output characteristic curve, that is, the so-called recording current curve.

  Further, since the coercive force Hc and the squareness ratio Rs of the magnetic layer 2 satisfy the relationship of the above formula (2), a sufficient reproduction output can be obtained without increasing the recording current value. An extremely useful magnetic recording medium capable of reducing the power consumption was obtained.

  Further, by setting the thickness of the magnetic layer 2 to 0.15 to 0.25 μm, a stable error rate margin was ensured, and the servo during the medium running in the magnetic recording apparatus could be performed stably. .

  Hereinafter, the magnetic recording medium of the present invention will be described in detail, but the present invention is not limited to the following examples.

FIG. 1 shows a schematic configuration diagram of an example of the magnetic recording medium of the present invention.
In the magnetic recording medium 10, the magnetic layer 2 is laminated on one main surface of the nonmagnetic support 1 via the nonmagnetic layer 3, and the back layer 4 is formed on the other main surface of the nonmagnetic support 1. It has a formed configuration. The magnetic recording medium of the present invention has a total thickness d of 12 μm or less in order to increase the recording density per volume.

In the magnetic recording medium of the present invention, the coercive force Hc of the magnetic layer 2 and the SFD (switching field distribution) have the relationship of the following formula (1), and the coercive force Hc of the magnetic layer and the squareness ratio Rs. Are characterized in that they have the relationship of the following formula (2).
220 ≦ Hc × (1 + 0.5 × SFD) (1)
2.2 ≦ Hc / Rs ≦ 2.6 (2)

Here, the SFD will be described with reference to the drawings.
FIG. 2 shows a hysteresis curve which is a magnetization curve of the magnetic material.
The relationship between magnetic flux density (B) and magnetic field (H) is
B = μ _d μ ₀ H (3)
Indicated by
Here, μ ₀ is the magnetic permeability in vacuum, and μ _d is the relative magnetic permeability in the medium. The differential coefficient related to the magnetic field of magnetic flux density is
μ _d = (1 / μ ₀ ) × dB / dH (4)
In this case, μ _d is called differential permeability.

On the other hand, the differential curve along the hysteresis curve becomes a loop having extreme values as shown in FIG.
If the half-value width related to this extreme value is ΔH, SFD is
SFD = ΔH / Hc (5)
Defined by
In the above formulas (4) and (5), the permeability, SFD, and magnetic field are related to each other by the differential curve and the full width at half maximum, and the permeability can be grasped from Hc and ΔH, that is, SFD, which can be easily measured. Is done. If magnetic particles are taken as the medium of the magnetic permeability, if attention is paid to the magnetic layer in terms of Hc fluctuations due to crystal defects such as the composition of the magnetic material and voids in the magnetic material, the filling rate of the magnetic particles in the layer and the particle number density The SFD is regarded as a parameter indicating the fluctuation of Hc, which is linked to the Hc fluctuation that controls the magnetic conditions such as the degree of dispersion.

The relationship between recording current and reproduction output in a magnetic recording medium is generally represented by a curve as shown in FIG.
In practice, the recording current at which the maximum value of the reproduction output can be obtained, so-called optimum recording current, is preferably low from the viewpoint of reducing power consumption, and is practically sufficient output (for example, an output value in a range 1 dB lower than the maximum output value). It is desirable that the width of the recording current (c in the figure) to obtain the above is wider.
In the present invention, focusing on the physical properties of the magnetic layer, Hc × (1 + 0.5 × SFD) and Hc / Rs are related to optimum recording current characteristics and recording current characteristics. The numerical range was specified as in the above formulas (1) and (2).

Next, each layer constituting the magnetic recording medium 10 of the present invention will be described in detail below.
As the material for forming the nonmagnetic support 1, any material that has been used as a support for a conventionally known magnetic recording medium can be applied. For example, polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate, polyolefins such as polyethylene and polypropylene, cellulose derivatives such as cellulose triacetate, cellulose diacetate, and cellulose butyrate, vinyls such as polyvinyl chloride and polyvinylidene chloride In addition to plastics such as resin, polycarbonate, polyimide, and polyamideimide, light metals such as aluminum alloy and titanium alloy, ceramics such as alumina glass, and the like.
When a rigid substrate such as an Al alloy plate or a glass plate is used as the nonmagnetic support 1, an oxide film such as alumite treatment or a Ni-P film may be formed on the surface and the surface may be cured. Good. Further, predetermined surface treatment such as corona discharge treatment, plasma treatment, easy adhesion treatment, heat treatment, dust removal treatment and the like may be performed in advance.

Next, the magnetic layer 2 will be described.
The magnetic layer 2 is composed mainly of a ferromagnetic powder and a binder, added with various additives such as abrasives, lubricants, dispersants, antistatic agents, etc., and magnetically adjusted by a conventionally known method using an organic solvent or the like. It shall be formed by applying a paint.

  As the ferromagnetic powder for forming the magnetic layer 2, for example, Fe-based and Fe-Co-based ferromagnetic metal powders are suitable. These may be used alone or in appropriate combination.

As the binder, any of known thermoplastic resins, thermosetting resins, and reactive resins can be applied as binders for conventionally applied magnetic recording media.
The blending ratio of the binder is 5 to 100 polymer parts, more preferably 10 to 30 polymer parts, with respect to 100 polymer parts of the ferromagnetic powder, from the viewpoint of dispersibility, adhesiveness, tackiness, and running durability during coating. Is preferred.
This is because when the amount of the binder is less than 5 parts by weight, dispersibility, adhesion, and running durability are impaired, and when it exceeds 100 parts by weight, tackiness and running durability are impaired.

The thermoplastic resins include vinyl chloride, vinyl acetate, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, acrylate ester-acrylonitrile copolymer, acrylate ester. -Vinyl chloride-vinylidene chloride copolymer, acrylic acid ester-vinylidene chloride copolymer, methacrylic acid ester-vinylidene chloride copolymer, methacrylic acid-vinyl chloride copolymer, methacrylic acid ester-ethylene copolymer, polyfluoride Vinyl, vinylidene chloride-acrylonitrile copolymer, acrylonitrile-butadiene copolymer, polyamide resin, polyvinyl butyral, cellulose derivative (cellulose acetate butyrate, cellulose diacetate, cellulose triacetate, cellulose Ropioneto, nitrocellulose), styrene-butadiene copolymer, polyurethane resin, polyester resin, amino resin, synthetic rubber, and the like.
Examples of the thermosetting resin include phenol resin, epoxy resin, polyurethane curable resin, urea resin, melamine resin, alkyd resin, silicone resin, polyamine resin, and urea formaldehyde resin.

In order to improve the dispersibility of the non-magnetic pigment, the molecules constituting the binder include —SO ₃ M, —OSO ₃ M, COOM, P═O (OM) ₂ (wherein M is a hydrogen atom or Represents an alkali metal such as lithium, potassium, and sodium), a side chain amine having a terminal group represented by -NR ₁ R ₂ , -NR ₁ R ₂ R ₃ + X-, and> NR ₁ R ₂ + X- Main chain type amine (wherein R ₁ , R ₂ and R ₃ represent a hydrogen atom or a hydrocarbon group, X − represents a halogen element ion such as fluorine, chlorine, bromine or iodine, an inorganic ion, an organic ion, etc. In addition, polar functional groups such as —OH—, —SH, —CN, and an epoxy group may be introduced. The amount of these polar functional groups introduced into the binder is preferably 10 ⁻¹ to 10 ⁻⁸ mol / g, more preferably 10 ⁻² to 10 ⁻⁶ mol / g.

Next, the abrasive will be described.
As the abrasive, various abrasives applied to conventionally known magnetic recording media can be used.
For example, α-alumina, β-alumina, γ-alumina, silicon carbide, cerium oxide, α-iron oxide, silicon nitride, titanium carbide, titanium oxide, silicon dioxide, boron nitride, fused alumina, silicon carbide, chromium oxide ( Cr ₂ O ₃ ), corundum, artificial corundum, diamond and the like.

Next, the lubricant will be described.
As the lubricant, dialkylpolysiloxane (alkyl is C1-5), dialkoxypolysiloxane (alkoxy is C1-4), monoalkylmonoalkoxypolysiloxane (alkyl is C1-5, alkoxy is carbon) 1 to 4), silicone oil such as phenylpolysiloxane, fluoroalkylpolysiloxane (alkyl is C1 to C5), unsaturated fatty acid hydrocarbon that is liquid at room temperature (n-olefin bipolymer bond is a terminal carbon) Examples thereof include a bonded compound, a carbon number of about 20), a fatty acid ester composed of a monobasic fatty acid having 12 to 20 carbon atoms and a monovalent alcohol having 3 to 12 carbon atoms, and fluorocarbons.
As the lubricant, fatty acid esters are particularly preferable. Examples of alcohols used as fatty acid ester raw materials include ethanol, butanol, phenol, benzyl alcohol, 2-methylbutyl alcohol, 2-hexyldecyl alcohol, propylene glycol monobutyl ether, ethylene glycol monobutyl ether, dipropylin glycol monobutyl ether, and diethylene glycol. Examples thereof include monoalcohols such as monobutyl ether and s-butyl alcohol, and polyhydric alcohols such as ethylene glycol, diethylene glycol, neopentyl glycol, glycerin, and sorbitan derivatives.
As fatty acids, acetic acid, propionic acid, octanoic acid, 2-ethylhexanoic acid, lauric acid, myristic acid, stearic acid, palmitic acid, behenic acid, arachidic acid, oleic acid, linoleic acid, linolenic acid, elaidic acid, plumitolein Aliphatic carboxylic acids such as acids or mixtures thereof.
Specific examples of the fatty acid ester include butyl stearate, sec-butyl stearate, isopropyl stearate, butyl oleate, amyl stearate, 3-methylbutyl stearate, 2-ethylhexyl stearate, 2-hexyldecyl stearate, Butyl palmitate, 2-ethylhexyl myristate, mixture of butyl stearate and butyl palmitate, butoxyethyl stearate, 2-butoxy-1-propyl stearate, dipropylene glycol monobutyl ether esterified with stearic acid, diethylene glycol Examples thereof include various ester compounds such as dipalmitate, hexamethylenediol esterified with myristic acid, glycerin oleate and the like.
In addition, to reduce hydrolysis of fatty acid esters that often occur when magnetic recording media are used at high humidity, branching / straight chain, cis / trans and other isomeric structures and branching positions of the starting fatty acid and alcohol are selected. To do.

The various lubricants as described above are preferably added in the range of 0.2 to 20 polymerization parts with respect to 100 parts by weight of the binder.
As the lubricant, the following other compounds can also be used. That is, silicon oil, graphite, molybdenum disulfide, boron nitride, zinc fluoride, fluorine alcohol, polyolefin, polyglycol, alkyl phosphate ester, tungsten disulfide and the like can be mentioned.

Next, the dispersant will be described.
Examples of the dispersant contained in the magnetic layer include caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, elaidic acid, linoleic acid, lauric acid, stearic acid, etc. metal consisting of fatty acids having 12 to 18 carbon atoms (R ₁ COOH, R ₁ is alkyl or Arunikeru group), an alkali metal of said fatty acid (Li, Na, K, etc.), or alkaline earth metals (Mg, Ca, Ba) Soap, fluorine-containing compound of the fatty acid ester, amide of the fatty acid, polyalkylene oxide alkyl phosphate ester, lecithin, trialkylyloxy quaternary ammonium salt (alkyl is 1 to 5 carbon atoms, olefin is ethylene, Propylene etc.) can be applied.
In addition, higher alcohols having 12 or more carbon atoms, and sulfate esters and the like can be used. These dispersants are added in the range of 0.5 to 20 polymer parts with respect to 100 polymer parts of the binder resin.
The lubricant may be added only to the magnetic layer 2, or may be added to both the magnetic layer 2 and the nonmagnetic layer 3, or only to the nonmagnetic layer 3.

Carbon black, graphite, metal powder, or the like may be added to the magnetic layer 2 as an antistatic agent in order to prevent adhesion of dust and dirt.
The content of the antistatic agent is usually 1 to 20% by weight of the ferromagnetic powder. If the amount of the antistatic agent is too large, the filling degree of the ferromagnetic powder is lowered, which is not preferable.

Next, the nonmagnetic layer 3 will be described.
The nonmagnetic layer 3 is composed mainly of a nonmagnetic powder and a binder, and may contain other lubricants, dispersants, conductive particles, and various additives. Any binder resin, lubricant, and dispersant for forming the layer 2 can be applied.

The nonmagnetic powder may have any shape such as a needle shape, a spindle shape, a rod shape, a cylindrical shape, a spherical shape, and a flat shape. The particle diameter of the nonmagnetic powder is appropriately selected depending on the applied film thickness.
When the nonmagnetic layer 3 is formed in a thin layer, it is preferable to use fine particles in order to ensure the smoothness of the surface, and those having a larger particle diameter can be applied as the thickness increases. For example, an acicular particle having an acicular ratio of about 10 may have an average major axis diameter of 0.05 to 0.4 μm, and a spherical particle may have a diameter of 0.01 to 0.1 μm.
The material of the nonmagnetic powder is not particularly limited, and any material such as metal, metal oxide, carbon black, and resin can be applied.
In addition, hematite (α-Fe ₂ O ₃ ) excellent in particle distribution and dispersibility is particularly preferable.

  The nonmagnetic layer 3 may also contain abrasive particles for forming the magnetic layer described above. The particle size of the abrasive is preferably 0.01 to 2 μm, but if necessary, nonmagnetic powders having different particle sizes can be combined, or even a single nonmagnetic powder can have the same effect by widening the particle size distribution. May be.

The non-magnetic particles contained in the non-magnetic layer 3 have a tap density of 0.3 to 2 g / cc, a water content of 0.1 to 5% by weight, a pH of 2 to 11, and a specific surface area of 1 to 30 m ² / g is preferred.
Specific examples of nonmagnetic particles contained in the nonmagnetic layer 3 include Sumitomo Chemical Co., Ltd., AKP-20, AKP-30, AKP-50, HIT-50, Nippon Chemical Industry Co., Ltd., G5, G7, S-1. TF-100, TF-120, TF-140, Ishihara Sangyo TT055 series, ET300W, Titanium Kogyo STT30, and the like.

The conductive particles in the nonmagnetic layer 3 are preferably carbon black, and rubber furnace, rubber thermal, color black, acetylene black, and the like can be used.
The specific surface area of the conductive particles is preferably 5 to 500 m ² / g, and the DBP oil absorption is preferably 10 to 1500 ml / 100 g. The DBP oil absorption amount of carbon black was determined by adding dibutyl phthalate to carbon black powder little by little, observing the state of carbon black while kneading, and finding the point of forming a lump from the dispersed state. The amount of addition (ml) is the DBP oil absorption.
The conductive particles preferably have a particle size of 5 to 300 μm, PH of 2 to 10, moisture content of 0.1 to 10 parts by weight, and tap density of 0.1 to 1 g / cc.
When carbon black is used as the conductive particles, Cabot Corporation, BLACKPEATLS 2000, 1300, 1000, 900, 800, 700, VULCANXC-72, Asahi Carbon Corporation, # 80, # 60, # 55, # 50, # 35, manufactured by Mitsubishi Chemical Industries, # 3950B, # 2400B, # 2300, # 900, # 1000, # 30, # 40, # 10B, manufactured by Columbia Carbon Co., CONDUCTEX SC, RAVEN 150, 50, 40, 15, Lion Exo Examples include Ketjen Black EC, Ketjen Black ECDJ-500, and Ketjen Black ECDJ-600.
Carbon black as described above may be surface-treated with a dispersant or the like, grafted with a resin, or part of the surface may be grafted.
In the nonmagnetic layer 3, the carbon black content is preferably 3 to 20% by weight with respect to the total nonmagnetic powder.
Carbon black as described above may be contained in the magnetic layer 2, and in this case, the amount based on the ferromagnetic powder is preferably 0.1 to 30% by weight.

  Carbon black has functions to improve the antistatic effect, friction coefficient reducing effect, light shielding effect, film strength improving effect, etc. of the magnetic layer 2 and the nonmagnetic layer 3, and these effects are applied to the carbon black to be applied. Can be appropriately controlled. Therefore, it is preferable to select carbon black according to the purpose based on the above-mentioned characteristics such as the type, amount, combination, particle size, oil absorption, conductivity, and PH.

In the step of forming the magnetic layer 2 and the nonmagnetic layer 3, the above compositions are mixed and dispersed with a kneading disperser to create a uniform coating solution. In addition, each material added to the magnetic layer 2 or the nonmagnetic layer 3 is not necessarily 100% pure. In addition to the main components, impurities such as isomers, unreacted materials, side reactants, dispersions, oxides, etc. May be contained. However, the impurities are preferably 30% by weight or less, and more preferably 10% by weight or less.
In addition, you may add all of each raw material or one part in any process which manufactures a coating liquid. For example, when mixing with a ferromagnetic powder before the kneading step, when adding at a kneading step with a ferromagnetic powder, a binder and a solvent, when adding at a dispersion step, when adding after dispersion, or when adding just before coating is there.

In addition, the following are mentioned as an organic solvent used when producing the coating material for magnetic layer 2 and nonmagnetic layer 3 formation.
For example, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, silohexanone, isophorone, and tetrahydrofuran, alcohols such as methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol, and methylcyclohexanol at an arbitrary ratio, Esters such as methyl acetate, butyl acetate, isopropyl acetate, ethyl lactate and glycol acetate, glycol ethers such as glycol dimethyl ether, glycol monoethyl ether and dioxane, aromatic hydrocarbons such as benzene, toluene, xylene, cresol and chlorobenzene Chlorination of methylene chloride, methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin, dichlorobenzene, etc. Hydrogen water, N, N- dimethylformamide, hexane and the like can be applied.
Note that these organic solvents are not necessarily 100% pure, and may contain impurities such as isomers, unreacted products, side reaction products, decomposition products, oxides, and moisture in addition to the main components. These impurities are preferably 30% by weight or less, and more preferably 10% by weight or less.

  The type and amount of the organic solvent are adjusted in the magnetic layer 2 and the nonmagnetic layer 3 as necessary. For example, a highly volatile organic solvent is used for the coating solution for the magnetic layer 2 to improve the surface property, or a coating solution for forming the nonmagnetic layer 3 is applied using a solvent having a high surface tension (cyclohexane, dioxane, etc.). However, the coating solution for forming the nonmagnetic layer 3 may be increased by using an organic solvent having a high solubility parameter, but is not limited to these examples. .

Next, the back layer 4 will be described.
The back layer 4 is prepared, for example, by using a conventionally known inorganic powder, a binder as a main component, and a dispersion containing various additives such as an antistatic agent, a rust inhibitor, and a curing agent using an organic solvent or the like. It is formed by applying paint.

  As the inorganic powder constituting the back layer 4, carbon black, various ceramic particles, metal oxides, and the like can be applied. Examples of the binder for the back layer include, but are not limited to, polyester-based polyurethane resins, phenoxy resins, and nitrocellulose resins, and those used as the binder for the magnetic layer described above. Either is applicable.

  Examples of the solvent for adjusting the coating material for the back layer include methyl ethyl ketone and toluene, but are not limited thereto, and any of the organic solvents for adjusting the magnetic layer coating material described above can be applied.

Next, the manufacturing process of the magnetic recording medium of the present invention will be described.
First, the nonmagnetic support 1 is prepared, and then the coating for forming the magnetic layer 2 and the coating for forming the nonmagnetic layer 3 are prepared.
The paint preparation process includes at least a kneading process and a dispersion process, and includes a mixing process of a predetermined material as necessary before and after these processes. Each process may be divided into two or more stages.

Ferromagnetic powders, binders, abrasives, nonmagnetic particles, antistatic agents, lubricants, organic solvents, and the like are added as appropriate at any stage of the coating preparation process. For example, polyurethane may be divided and added in a kneading step, a dispersing step, and a mixing step for year adjustment after dispersion.
In the kneading step, the residual magnetic flux density Br of the magnetic recording medium of the present invention can be increased by using a material having a strong kneading force such as a continuous kneader or a pressure kneader. When a continuous kneader or a pressure kneader is used, the ferromagnetic powder and the binder, or a part thereof (but preferably 30% or more of the total binding resin), for example, 100 parts by weight of the ferromagnetic powder The kneading process is performed in the range of 5 to 500 parts by weight.

The nonmagnetic layer 3 and the magnetic layer 2 constituting the magnetic recording medium of the present invention can be efficiently formed by a simultaneous multilayer coating method.
The nonmagnetic layer 3 and the magnetic layer 2 are applied by a wet-on-wet method in which a coating for the magnetic layer 2 is applied while the nonmagnetic layer 3 applied on the nonmagnetic support 1 is in a wet state. The method is preferably adopted. By this coating method, the adhesion of the magnetic layer 2 to the nonmagnetic layer 3 can be improved, and even when the magnetic layer 2 is a very thin layer having a thickness of 0.3 μm or less, peeling occurs between the layers. Therefore, it is possible to obtain a magnetic recording medium excellent in running durability in which dropout does not easily occur.
On the other hand, in the system in which the coating of the nonmagnetic layer 3 is applied and the magnetic layer 2 is applied after the drying process, it is difficult to obtain sufficient adhesion when the magnetic layer 2 is a very thin layer.

  As a specific method of the wet-on-wet coating method, as a first method, first, the nonmagnetic layer 3 is applied by a gravure coating, a roll method, a blade coating, and an extrusion coating device that are generally used in magnetic paints. While the layer is in a wet state, there is a method of applying the magnetic layer 2 with a non-magnetic support pressure type extrusion coating apparatus. As a second method, a coating head having a coating liquid passage slit built-in, There is a method in which the coating solution for the nonmagnetic layer 3 and the coating solution for the magnetic layer 2 are applied almost simultaneously. As a third method, the magnetic layer 2 and the nonmagnetic layer 3 are applied almost simultaneously by an extrusion coating apparatus with a backup roll. There is a method of applying.

  In order to prevent aggregation of particles dispersed in the coating liquid, it is desirable to apply a shearing force to the coating liquid inside the coating head. Also, what should be noted in the wet-on-wet coating method is the viscoelastic property (thixotropic property) of the coating solution. That is, if the difference in viscoelasticity between the coating liquids of the magnetic layer 2 and the nonmagnetic layer 3 is large, liquid mixture occurs at the interface between the magnetic layer coating layer and the nonmagnetic layer coating layer when the coating is applied. In the case of a magnetic recording medium having a very thin magnetic layer as described above, inconveniences such as deterioration of the surface properties of the magnetic layer 2 are likely to occur. In order to make the viscoelastic characteristics of the coating solution as close as possible, it is effective to first make the dispersed particles of the magnetic layer 2 and the nonmagnetic layer 3 the same. In the case of the magnetic recording medium of the present invention, Since the method cannot be adopted, carbon black is used as the nonmagnetic particles of the coating material of the nonmagnetic layer 3 in order to match the structural viscosity caused by the structure in which the magnetic particles are magnetically formed in the coating material of the magnetic layer 2. However, it is also effective to use non-magnetic particles having a small particle size other than carbon black. For example, by applying particles such as titanium oxide or aluminum oxide having a diameter of 1 μm or less, moderate aggregation is obtained, and a coating liquid having the structural viscosity of the particles is obtained.

Next, the back layer 4 is formed.
The back layer 4 is formed by applying a dispersion liquid containing various additives such as the inorganic powder, the binder, the antistatic agent, the rust inhibitor, and the curing agent described above using an organic solvent or the like. can do.

The total thickness of the magnetic recording medium 10 of the present invention is preferably as thin as possible in order to improve the recording density per unit volume. On the other hand, in order to ensure good head contact, the total thickness is 1 to 12 μm. Desirably, it is 7-12 micrometers.
An undercoat layer made of, for example, a polyester resin may be provided between the nonmagnetic support 1 and the nonmagnetic layer 3 in order to improve adhesion.

  As described above, the magnetic recording medium 10 shown in FIG. 1 has a configuration in which the nonmagnetic layer 3 and the magnetic layer 2 are laminated. However, depending on the purpose, the nonmagnetic layer 3 and the magnetic layer 2 are used. These physical characteristics may be changed. For example, the elastic modulus of the magnetic layer 2 is increased to improve running durability, and at the same time, the elastic modulus of the nonmagnetic layer 3 is made lower than that of the magnetic layer 2 to improve the contact with the magnetic head.

The magnetic recording medium of the present invention is suitable for high density magnetic recording.
In particular, in digital data recording used for writing and editing video signals, etc., high electromagnetic conversion characteristics are secured while maintaining good contact characteristics even for high-density recording with the shortest recording wavelength of about 0.3 μm. And it has the advantage that it is excellent also in durability in repeated use.

The configuration of the magnetic recording medium of the present invention is not limited to that shown in FIG. 1, and various structural additions may be added.
In addition, the nonmagnetic support 1 and the ferromagnetic powder and binder contained in the magnetic layer 2, the inorganic powder, binder and dispersant contained in the non-recording layer 3, other abrasives, antistatic agents, As the rust inhibitor and the solvent for adjusting the paint, any conventionally known solvents can be applied and are not limited to the examples described above.

  Hereinafter, the magnetic recording medium of the present invention will be described with specific examples and comparative examples.

[Example 1]
A magnetic layer dispersion having the following composition was prepared. First, the following materials were kneaded with an extruder.
Fe-Co ferromagnetic powder: 100 parts by weight (major axis length 0.1 μm, Co / Fe = 30 atm%, specific surface area 47 m ² / g,
(Saturation magnetization = 150 Am ² / kg, coercive force = 184 kA / m)
Vinyl chloride copolymer: 21.7 parts by weight (manufactured by Nippon Zeon MR-110: 30 wt% cyclohexanone solution)
Polyester polyurethane resin: 26.0 parts by weight (UR-8200 manufactured by Toyobo Co., Ltd .: 25 wt% methyl ethyl ketone / toluene mixed solution)
Next, the following materials were added to the kneaded product, stirred for 2 hours in a tank equipped with a disper stirrer, and then mixed for 5 hours in a sand mill.
Vinyl chloride copolymer: 11.7 parts by weight (manufactured by Nippon Zeon MR-110: 30 wt% cyclohexane solution)
Polyester polyurethane resin: 14.0 parts by weight (UR-8200 manufactured by Toyobo Co., Ltd .: 25 wt% methyl ethyl ketone / toluene mixed solution)
Methyl ethyl ketone (MEK): 88.0 parts by weight Toluene: 106.0 parts by weight Cyclohexanone: 32.7 parts by weight Thereafter, filtration treatment is performed using a pincushion type filter. First, using a filter having a filtration accuracy of 5 μm, a filtration process was sequentially performed using a filter of 3 to 1 μm.

Next, a nonmagnetic layer coating material having the following composition was prepared. The following composition was kneaded with an extruder.
α-Fe ₂ O ₃ (iron oxide) powder: 100 parts by weight (average major axis length 0.15 μm, specific surface area 55 m ² / g, average axis ratio 10)
Vinyl chloride copolymer: 21.7 parts by weight (manufactured by Nippon Zeon MR-110: 30 wt% cyclohexane solution)
Polyester polyurethane resin: 26.0 parts by weight (UR-8200 manufactured by Toyobo Co., Ltd .: 25 wt% methyl ethyl ketone / toluene mixed solution)
Next, the following composition was added to the kneaded product, stirred for 2 hours in a tank equipped with a disper stirrer, and then mixed with a sand mill for 5 hours.
Vinyl chloride copolymer: 11.7 parts by weight (manufactured by Nippon Zeon MR-110: 30 wt% cyclohexane solution)
Polyester polyurethane resin: 14.0 parts by weight (UR-8200 manufactured by Toyobo Co., Ltd .: 25 wt% methyl ethyl ketone / toluene mixed solution)
Methyl ethyl ketone (MEK): 61.3 parts by weight Toluene: 79.3 parts by weight Cyclohexanone: 19.3 parts by weight Thereafter, filtration is performed using a pincushion type filter. First, using a filter having a filtration accuracy of 5 μm, a filtration process was sequentially performed using a filter of 3 to 1 μm.

Each layer is formed using the magnetic layer forming coating material and the nonmagnetic layer forming coating material prepared as described above, and the following materials are added to each of the coating materials before the coating step, and a tank with a disper stirrer is added. For 30 minutes. The paint viscosity was 3500 cp for the magnetic layer forming paint and 7500 cp for the nonmagnetic layer forming paint.
Myristic acid: 1 part by weight Heptyl stearate: 2 parts by weight Isocyanate-based curing agent: 8 parts by weight (Coronate L manufactured by Nippon Polyurethane Co., Ltd., 50 wt% solid content)

  Further, the magnetic layer forming coating material and the nonmagnetic layer forming coating material were each filtered using a filter in a bobbin type having a filtration accuracy of 1 μm and an absolute filtration accuracy (absolute) of 10 μm.

As the nonmagnetic support 1, a polyethylene naphthalate film (PEN film) having a thickness of 7.5 μm is prepared.
On the nonmagnetic support 1, the nonmagnetic layer 3 was formed by simultaneous two-layer coating so that the film thickness was 2.5 μm and the magnetic layer 2 was 0.2 μm. Thereafter, magnetic field orientation was performed with a solenoid coil type magnet (5K Gauss), and a dryer drying process and a calendar process were performed with hot air at 100 ° C.

Next, a back layer 4 having a film thickness of 0.7 μm was formed by applying a paint having the following composition to the main surface opposite to the magnetic layer forming surface and performing a dryer drying treatment.
Carbon black (manufactured by Asahi # 50): 100 parts by weight Polyester polyurethane (N-2304 produced by Nippon Polan Co., Ltd.): 100 parts by weight Methyl ethyl ketone: 500 parts by weight Toluene: 300 parts by weight Cyclohexanone: 200 parts by weight Isocyanate curing agent: 40 parts by weight (Nippon Polyurethane Co., Ltd. Coronate L, solid content 50wt%)

  The magnetic tape produced as described above was slit into a 1/2 inch (12.65 mm) width and incorporated into a digital video tape recorder cassette (a digital beta cam cassette manufactured by Sony Corporation) as a sample.

[Example 2]
The Fe-Co ferromagnetic powder was changed as follows. Other conditions were the same as in Example 1, and a sample was prepared.
Fe-Co ferromagnetic powder: 100 parts by weight (major axis length 0.12 μm, Co / Fe = 30 atm%, specific surface area = 44 m ² / g,
(Saturation magnetization = 142 Am ² / kg, coercive force = 194 kA / m)

Example 3
The Fe-Co ferromagnetic powder was changed as follows. Other conditions were the same as in Example 1, and a sample was prepared.
Fe-Co ferromagnetic powder: 100 parts by weight (long axis length 0.08 μm, Co / Fe = 25 atm%, specific surface area = 55 m ² / g,
(Saturation magnetization = 127 Am ² / kg, coercive force = 205 kA / m)

[Example 4], [Comparative Example 1]
The magnetic properties were adjusted by changing the orientation conditions. The other conditions were the same as in Example 3 to prepare a sample.

[Comparative Example 2]
The Fe-Co ferromagnetic powder was changed as follows. Other conditions were the same as in Example 1, and a sample was prepared.
Fe-Co ferromagnetic powder: 100 parts by weight (major axis length 0.12 μm, Co / Fe = 30 atm%, specific surface area = 44 m ² / g,
(Saturation magnetization = 142 Am ² / kg, coercive force = 184 kA / m)

[Experimental Example 1]
The film thickness of the magnetic layer was 0.15 μm. The other conditions were the same as in Example 1, and a sample was prepared.

[Experiment 2]
The thickness of the magnetic layer was 0.25 μm. The other conditions were the same as in Example 1, and a sample was prepared.

[Experimental Example 3]
The thickness of the magnetic layer was 0.10 μm. The other conditions were the same as in Example 1, and a sample was prepared.

[Experimental Example 4]
The thickness of the magnetic layer was 0.30 μm. The other conditions were the same as in Example 1, and a sample was prepared.

  Each sample magnetic tape produced as described above was measured and evaluated under the following conditions. In addition, about [output frequency characteristic] mentioned later, measurement evaluation was performed about the sample shown in the said experimental examples 1-4.

〔output〕
Under a room temperature (25 ° C.) environment, recording of a single signal with a recording wavelength of 0.3 μm was performed by changing the recording current using a Sony digital VTR (trade name: HDW-500). Thereafter, signal regeneration was performed and the maximum value was measured.
In addition, the measured value was represented as a relative value (unit dB) with Example 1 used as a reference.

[Optimal recording current]
When measuring the output, the recording current when the maximum output was obtained was measured.
The numerical value is expressed in% by comparison when the optimum recording current of Example 1 as a reference is 100%.

[Recording current characteristics]
As shown in FIG. 5, a recording current curve showing the relationship between the recording current and the reproduction output is taken, and a line is drawn when it falls 1 dB from the maximum value of the reproduction output, and between the two points a and b crossing the recording current curve. Was measured. The numerical value was expressed as a relative value with the width of Example 1 as a reference being 1.

[Output frequency characteristics]
An output (AdB) with a recording wavelength of 0.3 μm and an output B with a recording wavelength of 1.2 μm were measured, and AB (unit: dB) was read.
The numerical value was expressed as a difference (unit dB) from the reference with reference to Example 1 as a reference.

The values of coercive force Hc, squareness ratio Rs, ratio Hc / Rs, SFD, Hc × (1 + 0.5 × SFD) of the sample magnetic tapes of the above [Examples 1 to 4] and [Comparative Examples 1 and 2] Table 1 below shows the measurement evaluation results for the output value (relative value), the optimum recording current (relative value), and the recording current characteristic (relative value).
The optimum recording current was evaluated as being practically good when the deviation from the reference (Example 1) was within ± 15%.
Further, regarding the recording current characteristics, if the deviation from the reference (Example 1) was within ± 20%, it was evaluated as practically good.

As shown in Table 1, the coercive force Hc of the magnetic layer and SFD (switching field distribution) have the relationship of the following formula (1). In the coercive force Hc of the magnetic layer and the squareness ratio Rs, In each of the magnetic tapes of Examples 1 to 4 having the relationship of the formula (2), the value of the optimum recording current necessary for obtaining a practically sufficient reproduction output is suppressed, and the evaluation of the recording current characteristics is also possible. The tolerance of the recording current value for obtaining a good and high output is widened.
220 ≦ Hc × (1 + 0.5 × SFD) (1)
2.2 ≦ Hc / Rs ≦ 2.6 (2)

From the measurement results shown in Table 1, FIGS. 6 and 7 show the relationship between Hc × (1 + 0.5 × SFD) and output, and the relationship between Hc × (1 + 0.5 × SFD) and recording current characteristics, respectively. Indicated.
According to this, it can be seen that the larger the value of Hc × (1 + 0.5 × SFD), the better the output and recording current characteristics.

For example, in Example 3 and Example 4, the recording current characteristic was 1.2 with respect to 1 of the reference (Example 1), and a high reproduction output was obtained in a very wide recording current range.
When the value of this recording current characteristic is large, even if the optimum recording current changes due to wear due to continuous use, the VTR magnetic head can exhibit an effect that a good error rate characteristic can be ensured, and durability as a product. The improvement of the property is achieved.

Next, the relationship between Hc / Rs and the optimum recording current (%) is shown in FIG.
According to this, it is understood that the deviation of the optimum recording current from the reference (Example 1) within ± 15% is in the range of 2.2 ≦ Hc / Rs ≦ 2.6.
In the sample of Comparative Example 1 in which the value of Hc / Rs exceeds 2.6, the optimum recording current value becomes as large as 124%, the deviation from the reference is large, and the power consumption for obtaining sufficient output is large. It has grown up.

  Also, the decrease in the value of Hc / Rs means that if Rs is made constant, Hc has to be lowered, which is a very disadvantageous condition for obtaining sufficient reproduction output, and particularly less than 2.2. Then, it was confirmed that the output characteristics deteriorated remarkably.

  On the other hand, in the sample of Comparative Example 2 where the value of Hc × (1 + 0.5 × SFD) is less than 220, the recording current characteristic is 0.7, the margin with respect to the error rate is reduced, and sufficient reproduction output is obtained. Therefore, the tolerance of the recording current value has been narrowed.

  Next, with respect to the samples of Experimental Examples 1 to 4 manufactured by changing the film thickness of the magnetic layer 2, the measurement result of the output frequency characteristic (comparison value with 4T output) and the evaluation result of the servo clock are shown in Table 2 below. Shown in

  In the samples of Experimental Examples 1 and 2 in which the thickness of the magnetic layer was in the range of 0.15 to 0.25 μm, the output frequency characteristics were maintained in the range of ± 3 dB, and practically sufficient characteristics were obtained.

  On the other hand, in the sample of Experimental Example 3 in which the thickness of the magnetic layer 2 is 0.10 μm, the output frequency characteristic is 5.5 dB, and since the magnetic layer 2 is too thin, a sufficient CTL signal reproduction output can be obtained. The servo lock was lost.

Further, in the sample of Experimental Example 4 in which the film thickness of the magnetic layer was 0.30 μm, the output frequency characteristic was −5.5 dB, and the output in the long wavelength region was significantly deteriorated.
As described above, when the output fluctuation becomes severe depending on the wavelength region, it is disadvantageous for the adjustment of the equalizer of the VTR, and there is a problem that the margin for the error rate becomes small as a result of being too large or too small.

1 is a schematic cross-sectional view of an example of a magnetic recording medium of the present invention. The magnetization curve and hysteresis curve for demonstrating the definition of SFD are shown. The differential curve for demonstrating the definition of SFD is shown. The relationship between recording current and output is shown. The figure for demonstrating the recording current for obtaining practically sufficient output is shown. The relationship between output and Hc × (1 + 0.5 × SFD) is shown. The relationship between the recording current characteristic and Hc × (1 + 0.5 × SFD) is shown. The relationship between the optimum recording current and Hc / Rs is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Nonmagnetic support body, 2 ... Magnetic layer, 3 ... Nonmagnetic layer, 4 ... Back layer, 10 ... Magnetic recording medium

Claims (2)

On one main surface of the nonmagnetic support, a nonmagnetic layer containing at least an inorganic powder and a binder and a magnetic layer containing at least a ferromagnetic powder and a binder are formed,
A magnetic recording medium having a total thickness of 12 μm or less formed by forming a back layer on the other main surface of the nonmagnetic support,
The coercive force Hc and SFD (switching field distribution) have the relationship of the following formula (1):
A magnetic recording medium, wherein the coercive force Hc of the magnetic layer and the squareness ratio Rs have a relationship of the following formula (2).
220 ≦ Hc × (1 + 0.5 × SFD) (1)
2.2 ≦ Hc / Rs ≦ 2.6 (2) The magnetic recording medium according to claim 1, wherein the magnetic layer has a thickness of 0.15 to 0.25 μm.

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